Onetailed

Onetailed is most commonly encountered as a misspelling of one-tailed, or as an informal shorthand in statistics for a one-tailed test. In statistical hypothesis testing, a one-tailed test is a directional test in which the alternative hypothesis specifies a direction of effect.

In a one-tailed test, the rejection region lies in a single tail of the probability distribution. The p-value represents the probability of observing a statistic as extreme as the observed one in the specified direction. The significance level alpha is allocated to that single tail, so only effects in the predicted direction can lead to rejection of the null hypothesis.

Choosing between one-tailed and two-tailed tests depends on the research question and prior hypotheses. A one-tailed test has more power to detect an effect in the chosen direction but cannot detect an effect in the opposite direction; if the true effect lies elsewhere, the test may fail to reject the null hypothesis. Therefore, the use of a one-tailed test should be justified a priori, rather than decided after looking at the data.

In practice, the term onetailed may also appear as a misspelling or as a proper noun in branding, usernames, or software names. There is no separate established meaning beyond the statistical sense, though context usually indicates whether it refers to the directional test or to something else.

See also: One-tailed test; Two-tailed test; Hypothesis testing.